This invention relates to methods for reducing furnace fouling in delayed coking processes, and more particularly in delayed coking processes in which the coker feedstock has a high propensity for furnace fouling.
Delayed coking is a non-catalytic thermal cracking process for treating various low value residual (“resid”) streams from petroleum refinery processes. The treatment enhances the value of such streams by converting them to lower boiling cracked products. In a conventional delayed coking process, as described for example in U.S. Pat. No. 4,455,219, feedstock is introduced to a fractionator to produce an overhead stream, a bottoms stream and at least one intermediate stream. The fractionator bottoms stream including recycle material is heated to coking temperature in a coker furnace. The heated feed is then transferred to a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes to form coke and volatile components. The volatile components are recovered and returned to the fractionator. When the coke drum is full of solid coke, the feed is switched to another drum, and the full drum is cooled and emptied by conventional methods. Alternatively, feedstock may be supplied to the coker furnace without first passing through a fractionator, as described for example in U.S. Pat. No. 4,518,487.
The delayed coking process employs a furnace that operates at temperatures as high as about 1000° F., roughly 50 to 100° F. higher than the operating temperature of the coke drum. The high furnace temperatures can promote the rapid formation of insoluble coke deposits on the furnace tubes and transfer lines. When coke deposits reach excessive levels, the operation must be shut down and the furnace de-coked. Frequent interruptions for cleaning can lead to high operating costs due to increased amounts of time the operation is off-line, in addition to the costs of the de-coking operations.
Although the process is referred to as “coking”, coke is often the least valuable product of the operation. Thus, it is often desirable to minimize the amount of coke produced and maximize the production of other cracked products such as coker gasoline, distillates, and various gas oils. One approach to the problem of furnace fouling is described in U.S. Pat. No. 4,455,219, wherein an internal recycle stream of volatile components produced during the coking operation (for example, flash zone gas oil, heavy coker gas oil, light coker gas oil, and coker naphtha) is substituted for part of the conventional coker heavy recycle stream. This approach reduces furnace fouling when using conventional coker feedstocks, but is not adequate to control furnace fouling when using coker feedstocks having a high propensity for furnace fouling. Although small amounts of feedstocks having a high propensity for furnace fouling can be blended in with conventional coker feedstocks, using them as the primary source of feed or blending them in large amounts with conventional feeds to a coker has not heretofore been feasible because of furnace fouling problems.
Thus, fouling of coker furnaces remains a costly problem.